High-entropy Alloys Research Articles

Research Progress of Pt-Based Catalysts toward Cathodic Oxygen Reduction Reactions for Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells (PEMFCs) have been considered by many countries and enterprises because of their cleanness and efficiency. However, due to their high cost and low platinum utilization rate, the commercialization process of PEMFC is severely limited. The cathode catalyst layer (CCL) plays an important role in manipulating the performance and lifespan of PEMFCs, which makes them one of the most significant research focuses in this community. In the CCL, the intrinsic activity and stability of the catalysts determine the performance and lifetime of the catalyst layer. In this paper, the composition and working principle of the PEMFC and cathode catalyst layer are briefly introduced, focusing on Pt-based catalysts for oxygen reduction reactions (ORRs). The research progress of Pt-based catalysts in the past five years is particularly reviewed, mainly concentrating on the development status of emerging Pt-based catalysts which are popular in the current research field, including novel concepts like phase regulation (intermetallic alloys and high-entropy alloys), interface engineering (coupled low-Pt/Pt-free catalysts), and single-atom catalysts. Finally, the future research and development directions of Pt-based ORR catalysts are summarized and prospected.

A novel Co-free high-entropy alloy with excellent antimicrobial and mechanical properties

A novel Co-free high-entropy alloy with excellent antimicrobial and mechanical properties

The influence of the cooling rate on the structure and corrosion properties of the multicomponent high-entropy alloy CoCrFeMnNiBe

The influence of the cooling rate on the structure and corrosion properties of the multicomponent high-entropy alloy CoCrFeMnNiBe

Carbothermal shock synthesis of FeCoNiPtRu high-entropy alloy for dual-function water splitting in alkaline media

The development of dual-function catalysts with high activity and durability is essential in overall water splitting applications. In this work, FeCoNiPtRu HEA was rapidly synthesized on carbon support by carbothermal shock process. The synergistic interaction among metal elements enhanced the catalytic activity. The catalyst demonstrated overpotentials of 104 mV for HER and 331 mV for OER at 10 mA cm−2. In addition, FeCoNiPtRu HEA was aimed on carbon paper and the nanoparticles were encapsulated by a carbon layer with a stability of up to 100 h. Finally, as a dual-function catalyst, when applied as both the cathode and anode in overall water splitting (OWS), FeCoNiPtRu demonstrated a voltage of 1.69 V at 10 mA cm−2. This work demonstrates the significant potential of high-entropy alloy catalysts in hydrogen production through overall water splitting.

Investigation of mechanical properties and high temperature wear resistance of CoFeNi1.5VZr0.4Six high entropy alloys optimized by Si alloying

The current work studies the mechanical properties and high-temperature wear resistance of CoFeNi1.5VZr0.4Six high entropy alloys by Si alloying. The Si alloying transforms Ni7Zr2 into a more wear-resistant silicide phase, and the microstructure changes from lamellar eutectic structure to dendritic structure. The high temperature microhardness, compressive strength and fracture toughness of the HEAs increase first and then decrease with the increasing Si content. The Si alloying in HEAs brings about significant improvements in wear resistance at elevated temperature through the combined effects of phase structure modifications and compositional changes in the tribo-layer. The augmented mechanical properties of the worn subsurface layer contribute to improved wear resistance at elevated temperatures.

Influence of He injection amount on the microstructure of ion irradiated Cr0.8FeMnNi medium entropy alloy compared with 316L stainless steel

In recent years, medium and/or high-entropy alloys (M/HEAs) have attracted attention due to their excellent mechanical properties, positioning them as potential candidates for structural materials in advanced nuclear reactors. This study investigated the effect of He injection amount on the microstructural evolution in Cr0.8FeMnNi MEA and 316L stainless steel. Helium atoms were injected, followed by Fe2+ irradiation. Transmission electron microscopy was used to characterize the irradiation-induced defects. The primary irradiation-induced defects observed across all He injection amounts were black dots; no voids were seen under any irradiation conditions. In He-injected Cr0.8FeMnNi, the number density of black dots was significantly lower than in non-He-injected Cr0.8FeMnNi, while the average size of black dots increased with higher He injection rates. As the He injection amount increased, the binding energy of the self-interstitial atom (SIA) to the He-V complex decreased. This reduction facilitated the nucleation and growth of numerous SIA clusters within the matrix, consequently increasing the number density of black dots. The change in the number density of black dots in Cr0.8FeMnNi was much smaller than that observed in 316L. This could be ascribed to the enhanced self-healing process caused by the diversity of point defects and higher lattice distortion in Cr0.8FeMnNi.

The effect of cooling rate on the precipitation behavior and fracture toughness of the η phase in LDED CoCrFeNiTi high-entropy alloy

Precipitation strengthening is recognized as an effective method for strengthening high-entropy alloys (HEAs) or medium-entropy alloys (MEAs), especially the recently developed CoCrFeNiTi HEA. The η phase plays a crucial role in strengthening CoCrFeNiTix HEAs, though it remains relatively understudied. In this study, CoCrFeNiTi HEA was fabricated by laser directed energy deposition (LDED). The research investigated the effect of cooling rate on the precipitation behavior and fracture toughness of the η phase in non-equilibrium solidified CoCrFeNiTi HEA. The findings revealed that the precipitation of the η phase in the as-deposited LDED sample was restrained, resulting in a very low volume fraction. However, a high-volume-fraction η phase was achieved through annealing at 900 °C for 6 h, followed by air cooling. This approach not only maintained the desired ∼6.2 % volume fraction of the η phase but also prevented its growth and coarsening (∼403 nm), resulting in an optimal combination of microstructure and mechanical properties. The η phase exhibited the Blackburn type orientation relationship with the FCC phase: <11 2‾ 0>η//<1 1‾ 0>FCC and {0001}η//{111}FCC. The L interface of the η phase demonstrated a coherent relationship with the FCC phase ({0001}η//{111}FCC), while the W interface was incoherent with the FCC phase ({11 2‾ 0}η//{1 1‾ 0}FCC). The unique interface characteristics promoted a higher growth rate at the W interface compared to the L interface, leading to the preferred growth of the η phase. The chemical compositions of both the FCC phase and η phase were accurately predicted by experiments and simulations, elucidating the precipitation kinetics of the η phase. When v ≥ vWC, the nucleation and precipitation of the η phase were inhibited. Conversely, when v ≤ vFC, significant coarsening of the η phase occurred. However, when v ≈ vAC, the AC sample exhibited superior mechanical properties, with a microhardness of ∼860 HV and excellent fracture toughness. These findings provide valuable insights and guidance for further enhancement and optimization of HEAs/MEAs strengthened by the η phase.

Ultrastrong, high plasticity, and softening-resistant refractory high-entropy alloy via stable isostructural coherent interfaces

Traditional approaches for improving the mechanical performance of alloys entail modifying interfaces, particularly grain boundaries, with elemental segregation or secondary phases. However, these methods face challenges in concurrently improving the strength, plasticity, and high-temperature softening resistance of alloys. Here, we uncovered that stable isostructural coherent interfaces effectively address these challenges. In the model body-centered cubic (BCC) MoTaVW refractory high-entropy alloy (RHEA) fabricated by mechanical alloying and spark plasma sintering, controlling the sintering temperature enhances the preferential segregation of W at interfaces. This results in a distinct BCC W-enriched nanolayer between micrometer-scale grains. This nanolayer facilitates dislocation slip and prevents grain growth, thereby improving both plasticity and resistance to high-temperature softening. Consequently, the MoTaVW RHEA featuring stable isostructural coherent interfaces achieves an ultrahigh yield strength of 1410 MPa and a plasticity of 22 % at ambient temperature. Even at 1200 °C, it maintains a yield strength of 575 MPa under hot compression.

Microstructure evolution and mechanical properties of a lamellar AlCoCrFeNi2.1 eutectic high-entropy alloy processed by high-pressure torsion

High-pressure torsion (HPT) was applied to a lamellar eutectic high-entropy alloy (EHEA) to study the effect of severe plastic deformation (SPD) on the composite structure and mechanical properties. We found that the existence of multiple phases affects defect distribution and the fragmentation process during HPT. Structural evolution shows orientation dependence with respect to the shear plane, which finally leads to a refined multiphase structure with nanograins and vortex clusters after a shear strain γ of 24. In nanograins, dislocation-mediated deformation prevails. The high density of grain boundaries, forest dislocations, and the generation of deformation twins restrict dislocation movement. As a result, an EHEA with a yield strength of 1.75 GPa, an excellent tensile strength of 2.20 GPa, and an appreciable failure strain of 5 % is realized. Our results demonstrate that the HPT deformation process of the lamellar EHEA is significantly affected by the structure. SPD on the multiphase structure is a suitable route for designing high-strength yet ductile alloys.

Laser lap joining of R60702 zirconium alloy and 304 stainless steel utilizing CoNiTiV high entropy alloy

In this study, the laser lap welding of R60702 zirconium alloy and 304 stainless steel was successfully achieved by incorporating CoNiTiV high entropy alloy (HEA) interlayer. The addition of HEA interlayer altered elemental diffusion and prevented the formation of Zr-Fe intermetallic compounds. The maximum tensile and shear strength of the joint (998.4 N) is 2.5 times that of the sandwich joint without the HEA layer (392.1 N), with the displacement also increasing by 85 %. The inclusion of the intermediate HEA layer transforms the original brittle fracture mode into a tough-brittle mixed fracture mode. The elimination of Zr-Fe intermetallic compounds is the primary factor contributing to the enhancement of mechanical properties. The main phase composition includes α-Zr, Cr2Zr, CoTi2, Ni7Zr2 and Zr3Co phase.

Efficient alloy design strategy for fast searching for high-entropy alloys with desired mechanical properties

The exponentially large compositional space of high entropy alloys (HEAs) offers more possibilities for designing alloys with desired properties. However, it also poses challenges to using the traditional “trial and error” approach in alloy design. In this work, an XGBoost model for predicting the elastic properties of the NbTiVZr family across the entire compositional space was established by combining density-functional theory (DFT) calculation results as the dataset with machine learning (ML) algorithms. Furthermore, considerations of charge transfer were incorporated into the solid solution hardening (SSH) model, and the model was further modified. Through comparing plasticity evaluation indices, the parameter D (γSurf/γGSFE) was determined to be suitable for predicting the plasticity of NbTiVZr alloys. A full compositional space model for yield strength and plasticity has been constructed based on the modified SSH model and the parameter D, respectively. Ultimately, an alloy design system combining the full compositional space models for yield strength and plasticity was established, achieving good consistency with experimental results. And a non-equiatomic alloy with a yield strength exceeding that of equiatomic alloys by 32.2 % (1409 MPa), while maintaining 29.27 % compressive strain was discovered. In conclusion, this work provides an efficient design strategy for alloys with desired properties.

Microstructure characterization and high-temperature wear mechanism of high-entropy alloy matrix composite coating fabricated by laser cladding

The development of diecasting technology requires hot die steels with high-temperature resistance. In this study, a high-entropy alloy (HEA) was combined with ceramic particles to generate a CoCrFeNiMo0.2Nb0.2/WC composite coating via laser cladding. The coating consisted of a face-centered cubic (FCC) matrix with unmelted WC particles, eutectics and nanoscale (Mo)NbC phases. With the strong lattice distortion caused by the sequestration of W atoms, a nanoscale amorphous layer existed between the Laves and HEA matrix phases. A transition layer with the same structure as the Laves phase was facilitated between the WC particles and the HEA matrix. Due to the hard phases in the HEA matrix and their interfaces, the composite coating with a high proportion of WC exhibited excellent wear resistance under high-temperature dry friction conditions. When the WC was 60 wt%, a minimum wear rate reached to 0.74 × 10−5 mm3/(N·m) at 600 ℃ and 1.46 × 10−5 mm3/(N·m) at 800 ℃. The destruction pattern of the composite coating at 600 ∼ 800 °C was due to the interaction of the adhesive, oxidative and abrasive wear. The composite coatings fabricated in this study provide a route to enhance the high-temperature wear resistance of die-casting molds.

Friction and wear properties of D-gun sprayed CrFeNiAl0.3Ti0.3-Ag-SrSO4 high entropy alloy matrix self-lubricating coating at elevated temperature

CrFeNiAl0.3Ti0.3-10 wt% Ag-(10−30) wt% SrSO4 high entropy alloy matrix self-lubricating coatings were fabricated using detonation spray technique and tribological characteristics of CrFeNiAl0.3Ti0.3-10 wt% Ag-(10−30) wt% SrSO4 coatings were explored from 25 to 1000 °C. The Ag and SrSO4 lubricants have good compatibility with CrFeNiAl0.3Ti0.3 matrix phase. The bond strength and hardness of CrFeNiAl0.3Ti0.3-Ag-SrSO4 coatings are 30 ∼ 65 MPa and 461 ∼ 535 HV, respectively. Meanwhile this coating exhibited good tribological performance across the wide temperature range and achieved the lowest friction coefficients of 0.29 ∼ 0.49 and wear rates of 2.68 ∼ 11.82 × 10−5 mm3/Nm when SrSO4 content is 10 wt%. Between 25 and 600 °C, SrSO4 and Ag lubricants, can effortlessly create complete lubricating films, preventing three-body wear and plastic deformation. At 800 and 1000 ℃, SrSO4 inflicts a significant impact on the formation of the oxide tribo-layer. When SrSO4 content is 10 wt%, it is feasible to form a compacted oxide layer containing SrCrO4, SrAl2O4 and SrTiO3, reducing the friction and wear. With further increasing SrSO4 content to 20 wt% and 30 wt%, the excessive SrSO4 leads to a detrimental on the compactness of oxide tribo-layer, increasing the friction and wear.

Carbon effect on tensile and wear behaviors for a dual-phase Fe61.5Cr17.5Ni13Al8 alloy

High entropy alloys (HEAs) series were produced by doping carbon into Fe61.5 Cr17.5Ni13-xAl8Cx (x = 0, 0.25, 0.5, and 1.5 at.%) by a vacuum arc melting. The carbon doping effect on the phase composition, mechanical properties, and wear behavior was studied. Fe61.5 Cr17.5Ni13Al8 alloy microstructure was a heterogeneous structure comprising FCC, BCC, and B2 phases. By introducing carbon into Fe61.5 Cr17.5Ni13-xAl8Cx (x = 0, 0.25, 0.5, and 1.5 at.%), the BCC phase fraction increased with decreasing FCC phase during annealing, accompanied by the formation of M23C6-like carbides at temperatures of 800 °C and 900 °C. The carbon-doped HEAs exhibited enhanced mechanical properties compared to the carbon-free HEAs, including higher tensile strength (1580 MPa), notable ductility (∼20 %), reduced friction coefficient (0.50), and improved wear resistance. Among the carbon-doped HEAs, the HEA with x = 0.5 at.% and annealed at 800 °C demonstrated exceptional wear resistance due to the compensations of attaining a balance between strength and elasticity.

Nano-dispersion strengthened and twinning-mediated CoCrNi medium entropy alloy with excellent strength and ductility prepared by laser powder bed fusion

Laser powder bed fusion (LPBF) was explored to produce CoCrNi medium entropy alloys (MEAs) from pre-alloyed powder with a relatively high oxygen concentration (870 ppm). Microstructure, strength, and toughness were investigated, revealing the effect of oxide dispersion strengthening (ODS). LPBF CoCrNi MEAs had a porosity of <0.10 % and superior mechanical properties compared to CoCrNi-based MEAs reported in the literature. Notably, the CoCrNi MEAs denoted as S1 and S2, exhibited an excellent combination of yield strength (>735 MPa), a UTS > 1 GPa, and a total elongation of ∼ 50 %. High strength results from the high number density of nano chromium oxides (average particle size ∼20 nm), which were in situ formed during LPBF, contributing to a calculated dispersion strengthening of ∼220 MPa. The deformation mechanism was investigated by TEM characterization of the specimen after tensile testing, revealing a large number of nano-twins for S1, cellular structure and nano-twins formed for S3. Extremely low porosity and deformation twinning ensure steady plastic deformation in LPBF CoCrNi, enabling high ductility. The prospect of ODS via LPBF provides a pathway for the strengthening and toughening of medium to high entropy alloys (HEAs).

Bio-corrosion behaviors and bio-compatibilities of TiNbZrTa and TiNbZrTaMo high entropy alloys

The bio-corrosion behavior and bio-compatibility of TiNbZrTa and TiNbZrTaMo high entropy alloys (HEAs) are crucial for their efficient maintenance during biological implantation. In this work, TiNbZrTa and TiNbZrTaMo HEAs were successfully prepared via vacuum melting, with a β-type titanium alloy Ti35Nb7Zr5Ta, exhibiting good biocompatibility used as a comparison. Due to the high entropy effect of equal atomic ratios, both TiNbZrTa and TiNbZrTaMo HEAs exhibit body-centered cubic (BCC) structure and lattice distortion compared to the β titanium alloy Ti35Nb7Zr5Ta. Mo elements contribute to the formation of a new BCC phase, resulting in a white matrix rich in Ta and Mo, and a dark second phase rich in Ti, Zr, and Nb. Electrochemical test results at 37 °C shown that TiNbZrTa has a wider and more stable passivation area than TiNbZrTaMo. The XPS results shown that formation of the passive film is related to the main elements added. Ti, Nb, Ta and Mo formed oxides via the solid-liquid interface and migrate inward, while Zr is formed at the passivation film/metal substrate interface and migrates outward, finally forming the layered structure of the passivation film. Additionally, the cytotoxicity test of mouse fiber cells was carried out to evaluate the biocompatibility of the HEAs. The results shown that the cell proliferation rate of TiNbZrTa and TiNbZrTaMo HEAs reached 0 and 1, respectively, with TiNbZrTa exhibiting better biocompatibility, as adding Mo reduced the cell proliferation rate. These findings may provide a method for predicting the bio-corrosion behavior and bio-compatibility of HEAs used for biological implantation.

Effect of Al–Zr and Si–Zr atomic pairs on phases, microstructure and mechanical properties of Si-alloyed (Ti28Zr40Al20Nb12)100-xSix(x=1, 3, 5, 10) high entropy alloys

The phases, microstructure, and mechanical properties of Si-alloyed (Ti28Zr40Al20Nb12)100-xSix(x = 1, 3, 5, 10) high entropy alloys are studied. Although silicides were not detected in the as-cast 1 at. % Si-alloyed sample, after annealing all the four alloys have the same phase composition: the Zr5Al3-type phase, Zr5Si3-type silicides, and the BCC/B2 dual-phase matrix, indicating that 1 at. % Si addition is adequate for the formation of silicides in equilibrium. The phase morphologies show a dependence on the Si content and the cooling rate, which changed from the occurrence of columnar grain zones together with coarse dendrites of the as-cast 1 at. % Si-alloyed alloy to four different cooling-rate-related morphologies in the other three as-cast alloys. After heat treatment, differently shaped precipitates tend to aggregate and spherize, and a three-step-shaped phase morphology is observed when the Si content is above 5 at. %. All precipitates are rich in Zr due to either the strong Si–Zr pair or Al–Zr pair, but heterogeneous distributions of Si and Al exist. An increase of the Si content led to a gradual decrease of the compressive strength in both as-cast and annealed samples, yet the total fracture strain does not show monotonously decrease trend. 5 at. % Si-alloyed samples possessed a higher total fracture strain than the 3 at. % Si-alloyed samples.

Mechanical properties of AlCoCrCuFeNi high-entropy alloys using molecular dynamics and machine learning

High-entropy alloys (HEAs) stand out from multi-component alloys due to their attractive microstructures and mechanical properties. In this investigation, molecular dynamics (MD) simulation and machine learning (ML) were used to ascertain the deformation mechanism of AlCoCrCuFeNi HEAs under the influence of temperature, strain rate, and grain sizes. First, the MD simulation shows that the yield stress decreases significantly as the strain and temperature increase. In other cases, changes in strain rate and grain size have less effect on mechanical properties than changes in strain and temperature. The alloys exhibited superplastic behavior under all test conditions. The deformity mechanism discloses that strain and temperature are the main sources of beginning strain, and the shear bands move along the uniaxial tensile axis inside the workpiece. Furthermore, the fast phase shift of inclusion under mild strain indicates the relative instability of the inclusion phase of hexagonal close-packed (HCP). Ultimately, the dislocation evolution mechanism shows that the dislocations are transported to free surfaces under increased strain when they nucleate around the grain boundary. Surprisingly, the ML prediction results also confirm the same characteristics as those confirmed from the MD simulation. Hence, the combination of MD and ML reinforces the confidence in the findings of mechanical characteristics of HEA. Consequently, this combination fills the gaps between MD and ML, which can significantly save time, human power, and cost to conduct real experiments for testing HEA deformation in practice.

HoDyGdCoAl high-entropy amorphous alloys working at full temperature range for hydrogen liquefaction with large magnetocaloric effects

Cryogenic magnetic refrigeration materials based on magnetocaloric effect can be used for hydrogen liquefaction. In this work, glass formation ability (GFA), magnetocaloric effect (MCE), refrigeration performance of melt-extracted HoDyGdCoAl amorphous ribbons have been investigated. Amorphous ribbons undergo a second order magnetic phase transition which achieve a table-like behavior broaden the range of magnetic transition temperature in hydrogen liquefaction.The maximum magnetic entropy change (-ΔSmaxM), the δTFWHM and the refrigeration capacity (RC) achieved 10.9 J/ (kg K), 84.2 K and 742.4 J/ kg (x = 5) and 11.2 J/ (kg K), 69.8 K and 643.0 J/ kg (x = 20) under a magnetic field change of 7 T, respectively. Combining wide magnetic transition temperature and large refrigerant capacity, the present amorphous ribbons can be used as potential magnetic refrigerant materials in hydrogen liquefaction temperature region.

Fragmentation /spheroidization of constituent phases and correlated flow softening during thermomechanical processing of AlCoCrFeNi2.1 eutectic high entropy alloy

The present study explores the morphological evolutions of constituent phases during the hot deformation process of a promising as-cast AlCoCrFeNi2.1 eutectic high-entropy alloy. Through which, a quasi-in-situ approach was employed using the Poliak-Jonas technique to carefully select interruption strains for conducting hot compression tests at 1000 °C. This approach allowed for tracking the sequence of microstructural evolutions. Furthermore, to understand the dominancy of shearing or thermally activated fragmentation mechanisms responsible for these evolutions, hot compression tests were conducted at different strain rates. Encountering an unexpected sharp softening behavior in the hot compression flow curve, its underlying micro-mechanisms were investigated using electron backscatter diffraction (EBSD) characterization and scanning electron microscopy (SEM) micrographs combined with detailed image processing and numerical calculations. It is shown that lamellar B2 eutectic regions underwent significant spheroidization with an increase in strain rate (from 12.6% ± 3.3%–64.5% ± 9%), while dendrites fragmentation was more pronounced in lower strain rates. These findings indicate that dynamic spheroidization and dendritic fragmentation mechanisms predominantly contribute to the aforementioned softening behavior observed in the alloy at high temperature.